1. Technical Field of the Invention
The present invention relates to a semiconductor energy detector effective for irradiation of energy rays such as UV rays, radiant rays, or particle rays which have very large absorption coefficients.
2. Related Background Art
Conventionally, a semiconductor energy detector disclosed in, e.g., Japanese Patent Laid-Open No. 6-29506 is known. Additionally, a semiconductor energy detector manufacturing method disclosed in, e.g., Japanese Patent Laid-Open No. 6-350068 is also known.
A charge-coupled device (CCD) sequentially transfers a charge cloud in one direction at a speed synchronous with a clock pulse. A CCD is a very ingenious functional device capable of converting spatial information into a time-series signal. However, to extract two-dimensional image information as a time-series signal, a certain device implementation is necessary. If charges are transferred while keeping the device irradiated with light, charges that are optically excited at the respective positions and transferred charges mix with each other to cause a phenomenon called smearing, resulting in degradation in video signal. To avoid this, the period (charge accumulation period) when the device is being irradiated with light and the period (charge transfer period) when charges are being transferred are temporally divided to do so-called time-divisional operation. Since the period when a video signal is output is limited within the charge transfer period, the video signal is intermittently output.
Generally, typical schemes of practical image sensing devices are frame transfer (FT), full-frame transfer (FFT), and interline transfer (IT). For measurements, the full-frame transfer (FFT) scheme is mainly used. This is because the full-frame transfer (FFT) scheme is suitable for a measurement of weak light since it has no accumulation region, and therefore the are of the photosensitive region is increased. In the full-frame transfer (FFT) scheme, however, since incident light is absorbed by charge transfer electrodes, the sensitivity considerably decreases for a short wavelength light with a large absorption coefficient.
The photosensitive region of a CCD that employs the typical full-frame transfer (FET) scheme has a structure in which the front surface of the photosensitive region is covered with transfer electrodes of polysilicon. Since polysilicon absorbs light with a wavelength of 400 nm or less, it cannot contribute to photoelectric conversion.
For some of such photodetectors, the photosensitive region of the substrate is thinned to about 10 xcexcm to 30 xcexcm to irradiate light from the back surface. When light is incident from the back surface, no obstacles are present on the back surface of the substrate except a thin oxide film. Hence, the sensitivity can be expected to become high for short-wavelength light. The back-illuminated CCD has a sensitivity to short-wavelength light of about 200 nm and can be applied even to an electron-bombard CCD device.
However, the saturation charge of a CCD is smaller than that of another image sensor. For this reason, if an object has a point with a high light intensity, generated signal charges spill over from a pixel to neighboring pixels. This results in a phenomenon that the image of the highlight portion looks like a several-time spread, i.e., blooming, and generated signals of the neighboring pixels are is lost.
Hence, to detect weak light immediately adjacent to strong light by a CCD for a spectroscope, an OFD (Over Flow Drain) for removing excess charges must be arranged to prevent any blooming of a signal of strong light.
An over flow drain applied to the frame transfer (FT) scheme or full-frame transfer (FFT) scheme is disclosed in, e.g., William Des Jardin and Stephen Kosman, xe2x80x9cTrue two-phase CCD image sensors employing a transparent gatexe2x80x9d, SPIE Vol. 3649, January 1999. A front-illuminated CCD is disclosed here, in which an over flow drain and a barrier region to the over flow drain are continuously formed on one side of each pixel.
In a conventional back-illuminated CCD as well, an over flow drain and a barrier region to the over flow drain are continuously formed on one side of each pixel, like the front-illuminated CCD. However, for the back-illuminated CCD, since a substrate portion where a vertical over flow drain is to be formed must be thinned by etching, no so-called VOFD (Vertical Over Flow Drain) can be formed. Even for a fully depleted CCD, since a substrate portion where a vertical over flow drain is to be formed is designed as a region for collecting charges, no vertical over flow drain can be formed. Hence, the back-illuminated CCD and fully depleted COD have no portions where a vertical over flow drain is to be formed. The back-illuminated CCD and fully depleted CCD therefore employ a so-called LOFD (Lateral Over Flow Drain). A three-phase CCD having a lateral over flow drain will be described with reference to FIGS. 1 to 4.
FIG. 1 is a plan view showing a conventional three-phase CCD having a lateral over flow drain. FIGS. 2 to 4 are sectional views taken along lines in FIG. 1. A vertical transfer channel 402 and isolation region 403 are formed on a p-type silicon substrate 401. A silicon oxide layer 404 is formed on the resultant structure. A plurality of vertical transfer electrodes 405 are formed on the silicon oxide layer 404. In this case, since the device is a three-phase CCD, three vertical transfer electrodes 405 are arranged. An over flow drain 406 and a barrier region 407 to the over flow drain are continuously formed on one side of each pixel. The over flow drain 406 and barrier region 407 to the over flow drain remove excess charges, thereby preventing blooming and smearing.
As described above, in the conventional CCD, the over flow drain 406 and barrier region 407 to the over flow drain are formed for all transfer electrodes in each pixel. Hence, especially in a back-illuminated CCD, since the lateral over flow drain is employed, the area of the region used to accumulate/transfer charges decreases due to the region where the over flow drain is formed. This decreases the amount of saturation charges of the CCD and reduces the aperture ratio.
The present invention has been made in consideration of the above situation, and has as its object to provide a semiconductor energy detector capable of improving the saturation charge and aperture ratio.
In order to achieve the above object, according to the present invention, there is provided a semiconductor energy detector having a region for detection and charge accumulation/transfer where a two-dimensional pixel array is formed on a surface of a semiconductor substrate on which energy rays become incident, characterized in that the region for detection and charge accumulation/transfer comprises a plurality of transfer electrodes formed in each pixel, and excess charge removing means arranged in correspondence with one of the transfer electrodes in each pixel.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.